This invention relates to an ionization device used, for example, as a part of a mass spectrometer.
A mass spectrometer generally operates such that sample molecules are ionized inside an ionization chamber and these ions are then separated and detected according to their mass numbers, or the mass per unit electric charge. The interior of the ionization chamber is usually maintained at a high degree of vacuum but residual gas components become attached to the inner walls of the chamber as the analysis continues, contaminating the chamber. In the case of ionization devices relying on a chemical ionization process, contamination is further caused by the reaction gas components which are introduced into the ionization chamber to be ionized.
If the interior of an ionization device becomes thus contaminated with undesirable components, the conditions for the generation of ions are adversely affected and the production rate of the ions is reduced. As a result, the sensitivity of the analysis may become lowered or the accuracy of the analysis may be affected because of the noise caused by such contaminants. In order to make it possible to carry out analyses dependably, it is necessary to disassemble the device and clean its components such that the contamination of the ionization chamber does not become too serious.
With prior art ionization devices, however, the degree of contamination cannot be determined from outside. Thus, the common practice has been to subject it to a cleaning process after analyses have been carried out for a specified length of time. As a result, it sometimes happens that an analysis is continued although the actual contamination has reached an unallowable level, especially in the case of a lengthy analysis, such that the sensitivity of the analysis drops in the middle of the work. It also happens sometimes that the device is disassembled for cleaning although the contamination has not reached an advanced level yet. This is a waste of both time and labor.